Christopher Daily-Diamond describes himself as a "dress shoe enthusiast." But the PhD student had a problem.
"I find that even when I tie them really tight, it just doesn't stay for as long as I'd like," Daily-Diamond told me. "I have to tie them multiple times a day and it was like 'why is that happening?'"
While other people might just gripe about a mundane problem like this, Daily-Diamond studies mechanical engineering at the University of California Berkeley. So rather than simply complaining, he decided to figure out exactly what was going on, and to find a potential solution.
There are a number of forces at play that contribute to the spontaneous untying of shoelaces, according to the study published in Proceedings of the Royal Society A on Tuesday. Daily-Diamond and his co-researchers were able to figure this out by recording a high-speed video of someone running on a treadmill until their shoelaces untied. From there, they were able to build a working hypothesis, which they tested with further experiments.
They found that the inertia created by swinging your feet forward whips the loops and loose ends of the laces forward. When combined with the impact of the shoe hitting the ground, the force of the two types of motion can loosen the base knot, and then the laces slip out.
Interestingly, the researchers found that neither the swinging motion nor the stomping motion alone were enough to untie the laces. You have to be walking or running to make things really come undone. The forces are surprisingly intense: through mounting a wireless accelerometer on different shoes and testing them, the researchers found that the laces experience an average of 7g of acceleration—that's roughly what Apollo spacecrafts endured on reentry to Earth—just from walking.
"That was a lot more than most people would think, and than we thought initially," Daily-Diamond said.
But the good news is there's a way to reduce how often your laces come undone: tie a better knot. Daily-Diamond and his colleagues tested a commonly-referenced modification to the traditional way we tie our laces (basically, you tie it the same way, but wrap the loose end around the loop in the opposite direction) and found that it stayed tied much longer than the normal knot.
The other option? Tuck your laces in to reduce the inertia acting on the knot. But the dress shoe enthusiast wasn't as partial to this solution. "In my opinion it's slightly dorky," Daily-Diamond said. "But it would definitely fix this problem."
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